Ctrl+Alt+Backspace Disabled

The Ctrl-Alt-Backspace key combination to force a restart of X is now disabled by default, to eliminate the problem of accidentally triggering the key combination. Users who do want this function can enable it in their xorg.conf, or by running the command dontzap --disable.

Enable Ctrl+Alt+Backspace Again

Ubuntu Linux 9.10 Karmic Koala

This part is added on the 23rd of November in 2009 as in the 9.10 version, enabling Ctrl+Alt+Backspace is different from doing it in the 9.04 version.
If you are using Ubuntu Linux 9.04 Jaunty Jackalope, please scroll down to skip this part then you can find one for Jaunty Jackalope.

This disallows the use of the Ctrl+Alt+Backspace sequence. That sequence is normally used to terminate the Xorg server. When this option is enabled, that key sequence has no special meaning and is passed to clients. Default: off.